PlasticFS may help, although I think (from reading, I haven't tried it) it allows you to effectively create a synthetic filesystem at some mount-point (kind of like FUSE, but using LD_PRELOAD).
So you can e.g create a synthetic root at
/tmp/myfs, but you would still need a chroot-like tool to present this constructed location as if it were the root of the filesystem. fakechroot is an obvious choice for not requiring root, but it also uses
LD_PRELOAD so I'm not sure if the two tools would confuse each other.
Update: It's a little confusing, but I've actually tried it now - versions of plasticfs up to and including 1.11 use LD_PRELOAD (and in fact include a "chroot" filter, so you don't need to use fakechroot). So that may be what you want, but:
it didn't work very well in my tests (created files it shouldn't, missed files that should have been there)
newer releases (1.11 was released in 2007, it's up to 1.14 now) abandoned LD_PRELOAD entirely for FUSE and a pair of
umount stateful commands, which also removed the ability to use it as a chroot.